Electronic control circuit for clothes dryers



y R. o. TR|PLETT 3,526,968

`ELECTRONIC CONTROL CIRCUIT FOR CLOTHES DRYERS Filed Jan. 12, 1968 Sept. 8,1970

5 Sheets-Sheet l Sept 8,1197() R. D. TRIPLETT 3,526,968?

' LEmc'raomc CONTROL VCIRCUIT FoncLoTHEs DRYERS Y Filed Jan. 12, -1968 5 sheets-sheet a sept. 8,' 1970 R. D. TRIPLETT l 3,526,968

ELECTRONIC CONTROL CIRCUIT FOR CLOTHES DRYERS I 3 Sheets-Sheet 3 Filed Jan. i2, 1968 ta u a J0 T no// w M 0 TX alu ,E2 T 6 I M T M m B l M n w w M H ,A m W L H f \v W I M United States Patent O lee U.S. Cl. 34-48 4 Claims ABSTRACT OF THE DISCLOSURE An electronic control circuit for an article dryer having one thermistor at ambient temperature and another thermistor at exhaust temperature, and a diierential amplier for monitoring the diiference in temperature Ebetween ambient air and exhaust air so as to respond to an increase in temperature `differential to initiate termination of dryer operation.

The present invention relates to a drying apparatus and more particularly to electrical controls for such apparatus.

One of the problems in operating drying apparatus, for example fabric dryers, is automatic termination of dryer operation when the articles are sufliciently dried. The appliance industry is especially competitive, and thus sophisticated electronic controls are not practical. However, because of the problem of relating temperature in a drying chamber to the moisture content of fabric, especially when the drying apparatus must operate at different temperatures, a need remains for versatile controls to terminate operation of the drying apparatus when the drying of articles has been accomplishd.

Accordingly, it is an object of the present invention to provide an improve-d electronic control for terminating operation of drying apparatus when suiiicient moisture has been removed from articles, for example fabrics, so that they are considered dry.

It is an overall object lof the present invention in accordance with the above to provide an improved electronic control for drying apparatus which is versatile, being responsive to operation of the apparatus in a range of environmental temperatures, yet is economical to manufacture and easy to maintain.

Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:

FIG. 1 is a side elevation, partially in section, of an exemplary drying apparatus embodying the present invention;

FIG. 2 is a front elevation of the drying apparatus of FIG. 1, with the top, front and drying chamber removed;

FIG. 3 is a plot of exhaust temperature v. time, showing the eifect of ambient temperature changes;

FIG. 4 is a schematic of a control circuit for the drying apparatus of FIG. 1 embodying the teachings of the present invention.

Referring now to the drawings and more specically to FIGS. l and 2, the domestic clothes dryer is provided with a cabinet having spaced side walls 11 and 12; a rear wall 13; top and bottom walls 14 and 15; and a front opening 19 of a rotatable clothes container or thereto and closing an opening, defined by a cylindrical ange 18, in the front wall 16, providing access to the front opening 10 of a rotatable clothes container or basket 20 for loading and unloading clothes in the basket.

The basket is mounted for rotation about a substantially non-vertical axis which, in the present case, is pref- 3,526,968 Patented Sept. 8, 1970 erably horizontal. More particularly, the basket is substantially cylindrical in shape, having a cylindrical wall 21; a front wall 22 having the opening 19 receiving the flange 18 of the cabinet front wall 16; and a rear wall 23 having a shaft and bearing assembly 24 for the basket 20 and a fan or blower 25. The basket is supported at its front end by spaced supports or skids 26, 26 mounted on the cabinet bottom wall 15 and having arcuate upper surfaces conforming to the curvature of the cylindrical wall 21 and provided With shoes 27, 27 of suitable bearing material engaging the basket. The rear end of the basket is supported by the shaft and bearing assembly 24 and, as seen in FIGS. 1 and 2, the basket rear wall 23 has conically-shaped depressed portions 29 providing a mounting means in the form of a sheet-metal hub 30 secured thereto as by welding and which hub receives a ball-like bearing secure to a shaft 32.

Turning now to a description of an air circulation system 25a, the blower or fan assembly 25 comprises a circular sheet-metal plate 33 having cut-out portions to define a plurality of radially-extending blades 34. Each blade is twisted from the plane of the plate, as Shown in FIGS. 1 and 2. The plate 33 is secured at its center to a pulley 35 rotatably mounted on shaft 32. The fan is conned between, and enclosed by, basket rear wall 23, particularly the conical portion 29 thereof and a front plate 36 of a duct assembly 37. A rear plate 38 of the duct assembly is spaced from the front plate 36 to provide a passage A therewith. The plate 38 is connected to bosses 39 (FIG. 1) formed in, and projecting from, the cabinet rear wall to provide passages B for the inflow of the ambient air. The fan draws ambient air through vents 40 in the cabinet rear wall 13, and forces air into the basket interior through a plurality of circumferentially and radially spaced openings 41 in the basket rear wall. As seen in FIG. l, the outer edge of the duct plate 36 is inwardly bent to provide a cylindrical flange 42 for mounting an annular felt seal 43, the seal engages the basket rear wall to confine air low between the basket rear wall and the plate 36.

Describing the exhaust portion of the air circulation system, the air flows through the basket into a plurality of openings 44 in an upper collector duct assembly 45 secured to the front walls of the cabinet. As seen in FIG. 1, the duct assembly 45 is a rectangular shaped box-like structure having front and rear walls `46, 47; side walls 48; and top and bottom walls 49 and 50, the front wall 46 being secured to the cabinet front wall, and the rear Wall 47 having the openings 44.

The air-circulation system further includes a second duct assembly 49 including a cylindrical conduit 50 connected to a funnel-shaped mernber 51 having a rectangular end receiving a complementary-shaped portion '52, dening the bottom opening of the duct assembly 44. The conduit 50 extends through the rear wall 13 of the cabinet and is connected, in conventional manner, to a standard vent or ue pipe for conducting the moistureladen exhaust to the outside of the house.

While ambient air is drawn into the dryer by the fan through the cabinet rear wall, an air heating assembly 53 comprising, in general, a cylindrical. housing 54, which may contain either a conventional electrically-energized heater 55, as shown, or gas-fired heating means is effective to heat air drawn into and circulated through the housing 54 by the operation of the fan.

The heating assembly 53 is supported on a pedestal P mounted on the bottom wall 15 of the cabinet. The housing 54 extends rearwardly and is connected to a triangular-shaped housing 55 (FIG. 2) forming portions of, and connected to, the front and rear plates 36 and 38 of the duct assembly 37, so that the fan is effective 3 to draw heated air in housing 54 and through housing 55 into the passage A for mixture of the heated air with the exterior ambient air drawn through passage B, the air mixture being forced into the basket through the openings 41 in the basket rear wall 29.

As exemplified in FIG. 3, it has been found that during the operation of a clothes dryer as the clothes are dried, there is a predetermined relationship between temperature and time. This is represented by plot 60 in FIG. 3. lInitially as hot air is directed into the clothes drying chamber or basket a substantial portion of the heat is used to warm-up the dryer (represented by time tl in FdG. 3). As the drying continues, substantially all the heat is utilized to vaporize moisture; the latter is represented by a plateau region, in FIG. 3 by the portion of plot 60 occurring during t2, 'wherein the exhaust temperature remains substantially constant. At the end of time period t1 plus t2 the clothes are substantially dry. As is clear from FIG. 2, the exhaust temperature begins to rise rapidly during a drying end time period t3. The abscissa represents normal ambient air temperature, thus it is clear that the differential between ambient air and exhaust air remains substantially constant, i.e. in the area of 70 to 80 F., during a steady state drying period or plateau period t2.

In accordance with the present invention, an improved electronic control circuit 61 is provided for sensing an increase in differential between intake or ambient air temperature and exhaust air temperature in the drying apparatus circulation system, which indicates that drying of the articles has been completed, and responding thereto by initiating termination of apparatus operation. Shown in FIG. 4 is the exemplary circuit 61 energized from an AC source -62 through a rectifying circuit 64, the latter providing a source of DC signal necessary for operation of the present circuit. The rectifying circuit 64 includes a diode 65', a resistor 66, and a capacitor 67. This is a half-Wave rectifying circuit with an RC lter well known to those skilled in the art. A voltage dropping resistor 68 is connected in series with the rectifying circuit 64 and reduces the DC signal tol the desired value and applies it to a junction 69. The DC signal appearing between junction 69 and a common conductor or bus 70 is applied to a circuit including relay coil 71 and an electronic switch 72, herein illustrated as an NPN transistor, and biasing resistor 74.

To provide a biasing signal for the electronic switch 72, a pair of biasing networks 75, 76 are provided. A lowered DC signal is provided by including a voltage dropping resistor 77 in series between junction 69' and a section junction 78. To regulate the DC signal, for reasons explained subsequently, a Zener diode 79 is connected between the junction 78 and the common conductor or bus 70. Turning to the electronic switch or transistor 72, the latter includes an emitter 72a, a base 72b and a collector 72e. The relay coil 71 is connected in series with the collector 72C and the emitter 72a through the resistor 74 to the common conductor or bus 70'. As shall be explained subsequently, the resistor 74 establishes a potential at emitter 72a, which for the NPN transistor must rbe exceeded by the base 72b potential in order for the transistor to conduct.

Describing the circuit for applying a voltage across resistor '74, the biasing network 75 includes an NPN transistor 81 having an emitter 81a, a base 81b and a collector 81c. The base is coupled to a slider 82 associated with a resistor 83.

'i The resistor 83 is part of a coupling circuit including a pair of resistors 84', 85. The resistors 83, 84 and 85 are all connected between junction 78 and common bus 70, across the Zener diode 79. Thus by movement of the slider 82 a selected portion of the DC signal appearing between junction 78 and bus 70 can be applied to the base of transistor 81 at 81b. The Zener diode assures a constant voltage supply for the biasing network so that positioning of the slider 82 provides an accurate selection of biasing signal. The collector-emitter or controlled circuit of transistor 81 includes a resistor `89 connected to junction 78, the collector 81e, the emitter 81a and the resistor 74. Accordingly, it is clear that as the slider 82 is adjusted so that conduction in the collector-emitter circuit of transistor 81 increases the voltage drop across the resistor 74 is also increased. The latter has the effect of increasing the voltage required at the base 72b of the transistor 72 for the latter to conduct.

Turning now to the second biasing network 76 which applies a signal to the base 72b of transistor 72, the latter includes a first negative temperature coefficient resistor, sold under the trade name Thermistor, connected in series with a second negative temperature coefficient resistor 91, both resistors 90, 91 being coupled across the regulated voltage appearing between junction 78 and bus 70. As shown in FIG. 2 the intake air temperature sensing resistor is located exemplarily on the dryer bottom wall 15 to respond to ambient temperature, and the exhaust air temperature sensing resistor 91 is located exemplarily on the wall of conduit 51. Of course, other locations for these temperature sensing and signal producing means may be used, it being necessary to sense a dilerential temperature between intake and exhaust as plotted in FIG. 3. For applying the temperature signal from the transducing resistors 90, 91 to the electronic switch, a junction 92 between the respective negative temperature coefficient resistors 90, 91 is connected through a current limiting resistor 94 to base 72b of transistor 72. Resistor 74 together with transistors 72 and 81 in the circuit connections described above form a differential amplifier.

summarizing the operation of the exemplary dryer control circuit, a rst value of current is permitted to flow while the intake air temperature sensing resistor 90 has a resistance value determined by ambient air temperature, and the exhaust air temperature sensing resistor 91 has a resistance value determined by a higher exhaust air temperature. Because the preset Voltage across biasing resistor 74, coupled to base 72b of transistor 72 through the resistor 94, is not enough to raise the potential of the latter above that preset at the emitter 72a, transistor 72 cannot conduct. The setting of slider 82 fixes the required differential temperature between intake or ambient air and the exhaust air to trigger the electronic switches 72.

Once the setting is made, then variations in ambient do not afr'ect the control. However, when the resistance of resistor 91 is lowered suiiciently due to the exhaust temperature increasing (shown in FIG. 3 as occurring during the time t3), current flow in the second biasing network 76 is increased sufficiently so that the signal at junction 92 biases transistor 72 to conduction. This can be understood, because as the resistance of resistor 91 decreases with temperature rise the current iiow through the substantially constant resistance ambient air resistor 90 increases. The resistance of resistor 90 remains substantially constant because room or ambient temperature is assumed to remain substantially constant. Accordingly, the voltage at junction 92 increases and the transistor 72 is biased to conduction.

When the transistor 72 conducts the relay coil 71 is energized and pulls in contacts 71a. The closing of the latter contacts completes a dryer cycle circuit 95 which controls the initiation of termination of dryer operation. Exemplarily this may involve turning Aoff the heater 55 while timing the continued circulation of air by letting the blower 25 operate for another tive-minutes or so.

As shown in FIG. 3, there are respective exhaust air temperature plots 60a, 60b for ambient higher than normal (dryer located in the garage of a southern United States home in midsummer) and for ambient tempera-` ture lower than normal (dryer located on the back porch of a northern United States home in midwinter). The respective negative coefiicient resistor elements 90, 91 respond, both to lower ambient and to lower exhaust, or to higher ambient and corresponding higher exhaust, to assure that electronic switch 72 is operated when the same predetermined dierence temperature between intake and exhaust is exceeded. This is automatic. Thus, the improved control circuit monitors the difference temperature between ambient and exhaust to respond by initiating termination of dryer operation when the predetermined temperature difference is exceeded, i.e., approximately 70 to 80 F.

While the invention has been described in connection with a preferred embodiment, it is understood that I do not intend to limit the invention to that embodiment.

I claim as my invention:

1. In a drying apparatus having an air circulation system including a chamber for drying articles, an air intake, a heater and an air exhaust, a circuit adapted to control operation of the drying apparatus responsive to a diterential temperature between the intake and the exhaust comprising in combination means coupled to the heater for initiating termination of the operation of the drying apparatus, an electronic switch coupled to said means, a rst and a second biasing network, respectively, cooperating to provide signals for turning on and turning olf said electronic switch, a pair of temperature responsive signal producing means included in said rst biasing network, a rst one of said pair at said intake station and a second one of said pair at said exhaust station, and means included in said second biasing network for adjusting the biasing network signal so that said electronic switch is operated after a predetermined temperature differential between the intake air and the exhaust air is exceeded to thereby initiate termination 0f operation of the drying apparatus.

2. In a drying apparatus having an air circulation system including a chamber for drying articles, an air intake, a heater and an air exhaust, a circuit adapted to control operation of the drying apparatus responsive to a differential temperature between the intake and the exhaust comprising in combination a first, negative temperature coefficient resistive means located to sense intake air temperature, a second, negative temperature coeiicient resistive means located to sense exhaust air temperature, a

differential amplifier including a irst normally conductive semiconductor means having an input and an output and a second selectively conductive semiconductor means having an in put and an output, and relay switch means coupled to said second semiconductor means output adapted to control energization of the heater, said rst and second resistive means, respectively, being coupled to said first semiconductor output and said second semiconductor input so that conduction by said first semiconductor means maintains a predetermined bias on said second semiconductor means dependent upon intake air temperature, said second resistive means effecting a change in current ow through said first resistive means to bias said second semiconductor means to conduction when exhaust temperature increases thereby indicating that a predetermined differential temperature between said intake and exhaust has been exceeded.

3. The combination of claim 2 including means fOr providing a D-C signal to the circuit including voltage regulating means for maintaining a substantially constant Voltage across a series circuit including said respective rst and second negative temperature coeicient resistive means.

4. The combination of claim 2 wherein said first and second semiconductor means, respectively, are rst and second transistors, respectively, with said first transistor being normally conductive until said exhaust air temperature responsive resistive means senses a predetermined rise in temperature between said exhaust air and said ambient air to bias said second transistor to conduction and said rst transistor to nonconduction.

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